Since youre gaining life while your opponents are losing life, Gray Merchant lets you pick and choose which of Astarions abilities you want to trigger at the end of your turn, either putting your opponents in a precarious situation, or helping you take the lead with a massive surge of health
The freerunning mechanic, in particular, offers a variety of effects at a steeply discounted price that makes the cards adequate for Modern play
And mechanically speaking, that shows up beautifully here as his static ability doubles all damage he would deal, and at the beginning of each end step, if he dealt damage to another Creature that turn, he gets a +1/+1 counter
Mind's Eye : Draw whenever an opponent does for 1 mana
Tireless Tracker is the whole package: You get more and more clues for the very low cost of playing your lands, and you get a payoff for cracking those clues